This collaborative research program investigates processes underlying the formation and tuning of complex sound categories. The overall goal is to provide a model of auditory categorization that can be readily applied to challenges of speech perception and communication disorders. Language learners form (phonetic) auditory categories of native-language sounds from the distributions of experienced speech sounds produced by many talkers. However, these averaged categories may not be appropriate for the speech produced by a specific talker. For example, non-native speech may not adhere to the patterns typical of native speakers. The aim of the current project is to develop and test a theoretical and practical model of how listeners use context to normalize, or tune, speech perception to the characteristics of a particular listening situation. The proposed experiments will move the model beyond mere demonstrations of normalization to make quantitative predictions of performance as a function of the content and temporal extent of the context. Such a practical model can be used to develop signal processing strategies for hearing aids and implants as well as to predict intelligibility of disordered speech. Building on the empirical outcomes of the previous project, the present research tests predictions arising from the hypothesis that a general auditory mechanism sensitive to the spectral interactions that occur between context and target sounds can account quantitatively for patterns of speech perception that appear to require extraction of vocal-tract-specific talker information. Another set of experiments will test the influence of perceptual learning of talker-specific patterns of speech in supporting this mechanism. A final series of experiments will bridge the gap that often exists between tests of speech perception phenomena and understanding real-world speech intelligibility and comprehension. Such a linkage is critical for deriving theory- and evidence-based clinical approaches in treatment of communication disorders.